The invention relates to a corrosion protected connecting device preferably for sheet metal components of light metal alloys primarily applicable in aircraft construction.
The sheet metal components to be connected with known types of connecting devices of the aforementioned type often have a high quality coating for avoiding corrosion. This type of coating is manufactured, for example, with the following process steps: the sheet metal components are pretreated by anodizing or they are covered with yellow chromate; an adhesive primer layer, for example, a phenol base primer layer, is applied and burned-in; a base layer with a polyurethane base is applied and possible burned-in; and finally, a protective layer, possibly also with a polyurethane base is applied. The nuts most typically used are made of steel and have a protective layer of cadium or silver for corrosion protection. A synthetic layer containing MoS.sub.2 is applied over the protective layer for providing a dry lubricant. The bolts used are manufactured of steel or titanium. The steel bolts have a cadium layer as corrosion protection. Basically, no corrosion protection is required for titanium screws. However, anodized titanium screws are also used.
The nuts are attached to the sheet metal components by riveting them on, riveting them in, or simply clamping them on. For reasons of production technology, the holes required for attaching the nuts are produced in most instances, only after the sheet metal components have already received their finish coat. Because of this, an electrical contact exists either directly or through the rivets between the sheet metal components and the nuts fastened thereto. The bores through the counterpart are also made after the finish coat has been applied, so that electrical contact of the sheet metal components with the bolts is possible here. Practically all of the participating structural elements have contact with each other since an electrical contact exists between the bolts and the nuts by means of the thread surfaces. The bolt connections described above are very susceptible to contact corrosion because of the very different chemical properties of the metallic materials used, e.g., steel cadium, titanium, and aluminum. The aluminum components are thus very strongly corroded at the points of contact with the adjacent structural elements made of different metals. The high quality finish coating mentioned above offers no protection against such corrosion because of the aforementioned reasons.
A known method of avoiding contact corrosion in bolt connections of the type mentioned, comprises applying a corrosion protective paint coating on the metallic bare surfaces of the bores in the sheet metal components before attaching the nuts or before assembly. This method has, however, the following disadvantages. First, the adhesion of this finish coat to the metal is insufficient because a suitable pretreatment of the outer surface of the metal is no longer possible at the time of use. Second, the application of the finish coat and the delay due to the drying time disturbs the production process.
Another known measure of the type mentioned comprises greasing all the components of the connection before assembly. Such method, however, has the following disadvantages: first, the contact of the components with one another is not interrupted by the grease, and second, contact corrosion sets in if the grease is removed by influences acting during the operation.
According to another known means for avoiding contact corrosion, the joints of the connection are filled during assembly with a rubber elastic curing sealing mass. Such method has the following disadvantages: first, the cleaning of the outer surfaces to be treated does not always meet the standards which must be set for attaining a good adhesion of the sealing mass to the structural elements, and second, contacts between the metal components may not be prevented with certainty as the mass is very soft in the beginning.
According to another known method of avoiding contact corrosion, flat intermediate layers made of electrically isolating material such as hard paper are arranged, for example, between the sheet metal components and the nuts which are to be riveted to the sheet metal components. The disadvantage of this process is seen in that, metallic contact of the bolts and rivets with the inner surfaces of the sheet metal bores are not avoided by these means.